17.3 Inheritance

2026 📋 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Describe inheritance as the transmission of genetic information from generation to generation
  2. Define an allele as an alternative form of a gene
  3. Understand and use the terms: dominant, recessive, phenotype, genotype, homozygous and heterozygous
  4. Use genetic diagrams, including Punnett squares, to predict the results of monohybrid crosses and calculate phenotypic ratios (1:1 and 3:1)
  5. Explain why observed ratios often differ from expected ratios, especially with small numbers of offspring
  6. State that two identical homozygous individuals that breed together will be pure-breeding
  7. Explain codominance using ABO blood groups (alleles I^A, I^B, I^o)
  8. Describe the determination of sex in humans using XX and XY chromosomes
  9. Describe a gene mutation as a random change in DNA base sequence, using sickle cell anaemia as an example
  10. Describe a chromosome mutation as a change in chromosome number or structure, using Down's syndrome as an example
  11. State that mutation, meiosis, random mating and random fertilisation are sources of genetic variation
  12. Understand that ionising radiation and some chemicals increase the rate of mutation

📖 Main Content

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1. What Is Inheritance?

Inheritance is the passing of genetic information from parents to their offspring, and from one generation to the next.

Think of it like this: the reason you may have your mother's eye colour or your father's nose shape is because genetic information was passed from your parents to you. This information is carried in your DNA, which is found inside the nucleus of every cell in your body.

  • DNA is organised into structures called chromosomes.
  • Chromosomes carry genes — each gene is a section of DNA that contains the instructions for a specific characteristic (like hair colour or blood group).
  • These instructions are passed down through reproduction, which is why children often look like their parents.

2. What Is an Allele?

A gene controls a specific characteristic. However, most genes come in slightly different versions. These different versions of the same gene are called alleles.

Allele = an alternative form of a gene.

Example: The gene for eye colour has different alleles — one allele may give brown eyes, and another may give blue eyes. Both are versions of the same gene (eye colour), but they give different results.

  • Every person has two alleles for each gene — one inherited from their mother and one from their father.
  • These two alleles may be the same, or they may be different.

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